1. Field of the Invention
This invention relates to imaging systems where the image resolution and image scale do not vary over a range of object distances, for example as may be used in systems that image human irises.
2. Background
As traditional forms of personal identification become vulnerable to advancing technology, biometric identification is increasingly seen as a viable approach to personal identification. Techniques such as voice recognition, fingerprinting, and iris imaging rely on physical personal traits that are difficult to change or duplicate.
Biometric identification via iris imaging relies on capturing images of a person's iris and then matching the captured image against a database of previously acquired iris information. In order for this approach to be effective, the subject's iris image must be captured with a certain amount of resolution. Low resolution images do not provide enough information to uniquely match against the database. Furthermore, in many applications, it is desirable to capture iris images from many subjects quickly, without requiring excessive user cooperation, and from a distance.
For example, it may be desirable to capture iris images of every person funneling through an entranceway in an airport. Thus, an iris imaging system may continuously scan a certain volume in front of the entranceway (the capture volume), capturing iris images of the people in that volume. Larger capture volumes are generally preferred.
However, larger capture volumes mean that the subjects will be located at widely varying object distances from the camera capturing the iris images. In a conventional imaging system, the back focus distance grows as the object moves closer to the lens. This results in an increased optical magnification which means that the size of the image formed at the camera's image plane (i.e., the image scale) varies with the object distance. The same size iris will produce an image that is larger when the subject is closer to the camera and smaller when the subject is farther from the camera. In addition, the resolution of the camera, when measured in units of length at the image plane, may also vary with the object distance. This complicates the subsequent image processing. The varying scale means that the captured image must be rescaled to properly match the iris images in the database. The varying resolution means that features used to distinguish one iris from another may be blurred relative to the iris images in the database. The varying image size means that the image detector must be sized to accommodate the largest image, which can drive the use of a significantly larger image detector than would be needed for a fixed focus device.
Thus, there is a need for iris imaging systems that eliminate or at least reduce this image scale and/or image resolution variation as a function of object distance to the subject.